The Ark

The Ark, also known as Installation 00, is a Forerunner structure discovered in Halo 3. The Ark can only be accessed through the Portal by using a Forerunner ship 'Dreadnought'; a keyship. The Ark has multiple functions. Since it is located far outside the of the Milky Way Galaxy (as you can tell by looking at the sky while on it), it serves as a safe haven from the damage radius created by the Halos. It also has the ability to create new Halos, as seen at the end of the level The Covenant. It also has the ability to fire off all the Halos at once, as told by 343 Guilty Spark at the end of Halo 2.

At the end of Halo 3, the Ark is heavily damaged when the Master Chief activates the newly constructed Installation 04 Halo prematurely, thus destroying both Installation 04 (2) and The Ark.